Piezoelectric crystal unit



Jan. 11, 1949. FRUTH ETAL 2,458,987

PIEZOELECTRIC CRYSTAL UNIT Filed July 18, 1945 I 2 Sheets-Sheet l IN 5 EN TORS.

Hal]? [9142224 BY 1Z0 Ji dercg Patented Jan. 11, 1949 UNITED STATES PATENT OFFICE 2.45am I PIEZOELEOTBIC cars'rar. UNIT Hal F. Fruth. Chicago, and Roger 1. Pierce, Park Ridge. 111., auignors to Motorola. Inc., a corporation of Illinois Application July 18, 1945, Serial No. 605,782

13 Claims. (Cl. 171-327) The present invention relates to piezoelectric crystal units of the character employed for freeither by reactance tube control of a conventional tuned circuit oscillator or by providing separate modulator and crystal controlled oscillator stages and effecting the desired signal modulation of the oscillatorfoutput frequency in the modulator stage. Systems of the latter type, while p08- sessing the frequency stability inherent in crystal controlled systems, are relatively costly due to the amount of apparatus involved. Systems of the first type are subject to objectionable and uncontrolled drift in the output center or carrier.

frequency of the signal modulated carrier.

It is an object of the present invention, therefore, to obviate the above diiiiculties by providing an improved piezoelectric crystal unit, the output frequency of which may be directly and accurately controlled in accordance with a variable voltage impressed thereon. 1

It is another object of the invention to provide an improved variable frequency piezoelectric crystal unit having an output frequency which is not appreciably affected by ambient temperature changes.

It is a further object of the invention to provide improved and exceedingly simple facilities for varying the output frequency of a piezoelectric crystal.

More generally, it is an object of the invention to provide a variable frequency. piezoelectric crystal unit which is of small, rug ed'and low cost construction, and when combined with the usual circuit elements of a piezoelectric oscillator, will reliably function as an oscillator-modulator having great center frequency stability.

The invention, both as to its organization and method of operation, together with further obiects and advantages thereof, will-best be understood by reference to the following specification taken in connection with the accompanying drawings, in which:

I Fig. 1 is an explosion view illustrating the components of an improved variable frequency piezoelectric crystal unit characterized by the features of the present invention;

- Fig. 2 is a side viewin full section illustrating the unit when the parts thereof are completely assembled;

Fig. 3 is a top view in perspective of the diaphragm subassembly embodied in the unit;

Figs; 4 and 5 are graphs illustrating certain operating characteristics of the unit; and

Fig. 6 is a view schematically illustrating the unit as an oscillator-modulator in a frequency modulation wave transmission system.

Referring now to the drawings and more particularly to Figs. 1, 2 and 3 thereof, the present improved crystal unit is there illustrated as comprising a piezoelectric quartz crystal ill of rectangular configuration and having a thickness determined by the particular frequency at which the crystal is designed to oscillate. In accordance with the present invention this crystal is clampingly supported between a thermally deformable bimetallic electrode l I, having the function of compensating the crystal output frequency against drift occasioned by ambient temperature changes, and a deformable magnetic diaphragm electrode I2 which is variably deformable under the control of electro-responsive means indicated generally at l9 to vary the output frequency of the crystal in accordance with an applied-control or signal voltage. The electrode ll, together with the crystal Hi and the diaphragm driving unit I9, are adapted to be received within an open cavity Ha formed within an insulating housing member H which is preferably comprised of a suitable plastic or ceramic material. Along one edge thereof, this housing member is provided with a pair of holes lid and He within which the connecting pins 16a and Illa of conductive terminals [6 and 20 are respectiv'ely disposed. Preferably these pins, which are adapted for mating engagement with the contacts of a suitable socket, are embedded or otherwise sealed into the opemngs Nd and He through which they respectively extend. At its inner end, the conductor l8 terminates in a flat plate llb which is adapted to .be pressed into engagement with the upper surface of the electrode H. The wire extension of the conductor 20 is adapted for soldered connection" with a conductive non-magnetic housing II which supports the parts of the the periphery of the diaphragm l2. Thus the 3% housing member 29 functions as a holder for the diaphragm i2. A split ring i911 ciampingly embracingthe housing member at externally of the housing M is employed to maintain the unit i9 assembled with the housing id.

For the purpose of preventing relative rotary movement between the two elements it! and H and the housing It, the cylindrical cavity Ma is provided with corner receiving recesses Mb within which the corner portions of the two named elements are receivable. opposite faces of the crystal In from the electrode ll andthe diaphragm l2, the electrode II is provided with downwardly depending corner parts or feet Ila which bear against the corner' portions of the upper crystal face, and the upper edge of the housing member 2| is cut away to form four equal angularly displaced feet 2lb which bear against the lower crystal face. For the purpose of urging the electrode I I toward the housing member 2 l, thereby to clamp the corner portions of the crystal between the feet Ila and 2th, a helical coil spring l8 isprovided which, during the assembly of the unit, is compressed between the upper terminal plate I6 and an insulating cover l which is utilized to close the open side of the housing l4. Assembly screws II and nuts Ila extending through registering openings in the housing l4 and the cover I5 are employed to maintain the two named parts in assembled relationship.

As best shown in Fig. 2 of the drawings, the

In order to displace the assess? diaphragm 62. With the driving unit i9 thus assembled with the housing id, the crystal it) is matched with a bimetallic electrode ii in the manner more fully described below, following which the matched crystal and electrode are inserted beneath the conductor plate 55 for support by the feet Zlb of the housing member M The spring I8 is next placed in position over the terminal plate! 6 and the cover pressed down against this spring until its edges are in meeting engagement with the edges of the housing M. The assembly screws l1 and nuts Ila may now be employed to secure the cover i5 and housing M together, thuscompleting the assembly of tr unit. I

From the preceding explanation it will be apparent that the crystal Ill is positively restrained driving unit I9 comprises a magnetic field structure which may be variably excited to distort the diaphragm l2 toward and away from the lower crystal face for the purpose of changing the output I frequency of the unit. In brief, this field structure comprises a permanent field magnet 22 having a .pole face end 22o adjacent the under surface of the diaphragm and having a laterally extending part 22b which carries a center pole piece 23 having a pole face end 23a disposed adjacent the under surface of the diaphragm I! in the exact center thereof. Preferably, the permanent magnet 22 is formed of Alnico or the like and, in cooperation with the pole piece 23, serves normally to bias or distort the diaphragm l2 to its normal setting thereof relative to the lower face of the crystal to. In order to increase or decrease this bias, a driving coil or winding 24 is provided which embraces the center pole piece 23 and is adapted for energization from a connected control or signal voltage source. The components of the described field structure are supported within the housing member 2 I upon an end closure plate 26 which is formed of Bakelite or another suitable insulating material and is rigidly interconnected with the lower end of the member 2| in any desired manner. This closure plate carries a pair of terminals 25 which are connected to the terminal ends of the winding 24 and through which connections may be made to a control or signal voltage source.

In the assembly of the described components of the umt, the housing I and terminals l6 and 20 are preassembled as a separate subassembly. Similarly the components of the diaphragm driving unit I9 are preassembled as a separate subassembly. These two assemblies are brought together by inserting the tubular housing member 2| through the opening c at the bottom of the cavity Ma and telescoping the split ring I 9a over the housing member 2|. The next step is that of soldering theterminal conductor 20 to the rim of the housing member 2| to provide an electrical connection between the socket pin 20a and the against lateral motion relative to the electrode II and the diaphragm 12 by virtue of the spring developed clamping pressure exerted upon the corner portions thereof by the parts Ma. and 2"; between which the crystal is held. Further, the seating engagement of the crystal and electrode corner parts with the sides of the recesses Mb precludes any appreciable rotary movement of the crystal relative to the electrodes. In this regard it will be noted that only a very small portion of the crystal face area is held under compression between the parts H a and 2th, thus minimizing the decrease in crystal activity resulting from the restraining force exerted through the thickness of the crystal. With this construction, and as best shown in Fig. 2 of the drawings, an air gap I lb is provided between the lower surface of the bimetallic electrode II and the upper face of the crystal. A similar air gap I2b is provided between the lower crystal face and the diaphragm l2. These two air gaps, due to the deformable characteristics ofthe electrodes Hand l2 are each variable to determine the output frequency of the crystal unit.

In production, quartz crystals may be so cut that the output frequency of each crystal usually varies directly with temperature, 1. e. as the crystal temperature decreases the natural frequency of vibration decreases and vice versa. Another,

important and well known factor in determining the output frequency of a crystal unit is the mass, density and elasticity of the air between the crystal and the electrodes associated with the two faces thereof. This factor is, of course, determined by the area and length of each of the air gaps Nb and l2b between the crystal faces and the two electrodes. To a lesser extent, the interelectrode capacitance between the electrodes I l and I2 affects the output frequency of the crys-,

tal, which capacitance is varied in response to expansion and contraction of the crystal through the thickness thereof resulting from 'changing temperature of the crystal. In general, and for an air gap of given area, thetwo last-mentioned variable factors are changed to produce an increase in the crystal output frequency as the air gap length is increased and vice versa.

Another factor which in part determines the temperature-output frequency characteristic of the illustrated crystal unit is the temperature response of the diaphragm l2 and its associated driving unit l9. For example, the experimentally determined curve B illustrated in Fig.

,4 of the drawingsyrepresents the temperatureoutput frequency characteristic of a substantially perfect crystal as tested using conventional nontemperature responsive electrodes in association therewith. When. however, the diaphragm l2 ally decreased to a value approximately 100 cycles less than the rated frequency value of the crystal. While this modification of the crystal temperature output frequency characteristic may be attributable to several factors,-it is probably in part produced by the changing air gap unit ll were substituted assume? exactly counterbalances the tendency of the diaphragm and associated driving unit to change the output frequency of the crystal in the opposite sense, such that a resilient characteristic of substantial uniformity is obtained.

' If, on the other hand, a crystal II is used having the more usual temperature-output frequency characteristic F when tested in con- Junction with conventional pressure type elecbetween the diaphragm armature Ila and the associated pole faces of the magnet 22 and the pole piece 23, occasioned by unequal expansion of the housing member fl and the named parts of the field structure. Such change in the field structure air gap dimensions produces a decrease in the attractive force exerted upon the diaphragm armature lid to effect a decrease in the length of the air gap lib and hence an increase in the crystal output frequency.

'In order to reduce the variations in crystal output frequency which are produced in the above described manner in response to ambient temperature changes, the bimetallic thermally deformable electrode ll isassigned the function of varying the air gap l lb in the correct sense to compensate for the output frequency drift which wouldotherwise occur, i. e. to render the temperature-output frequency characteristic of the crystal as nearly uniform as possible. To i this end, the electrode II is arranged so that the center region thereof is cupped toward or away from the upper crystal face to correspondingly reduce the average length of the air gap H1; in response to changing temperature.

The direction of cupping must initially be determined by the type of compensation required. Thus, if it is desired or necessary only to correct the frequency drift represented by the characteristic C in Fig. 4 of the drawings, the high coefllcient of expansion side of the electrode II is disposed away from the ,upper crystal face so that as the ambient temperature rises to produce a corresponding rise in the temperature of the crystal Ill and the other parts of the unit, the center region of the electrode II is cupped away from the upper crystal face effectively to increase the average length of the air gap llb. Conversely, as the temperature decreases from the value at which the electrode is not deformed, the center region of the deformable electrode ll is' cupped toward the upper face of the crystal to effectively decrease the average length of the air gap llb.

The degree of deformation of the electrode ll thus produced in response to a given change in the ambient temperature is, of course, determined by the amount of departure of the temperature-output frequency characteristic of the crystal and associated diaphragm unit from unlformity. In the case considered above wherein a temperature-output frequency characteristic C is obtained using a non-deformable electrode trodes, partial compensationis obtained by associating the crystal with the diaphragm II and associated driving unit. Thus, by subtractin the values along the curve C from the corresponding values along the curve F, a resultant characteristic curve E may be obtained which is more nearly uniform over the indicated temperature range than the characteristic curve 1'. To further compensate the combination so that the characteristic is substantially exactly uniform, it is necessary to employ a thermally deformable electrode II, the action of which is the exact reverse of that described above for a crystal initially having a substantially uniform temperature-output frequency characteristic. Specifically, an electrode ll is used in which the high coefllcient of expansion side thereof is disposed adjacent the upper face of the crystal Ill so that as the ambient temperature rises from the value at which the electrode ll is undistorted, to produce a corresponding rise in the temperature of the crystal Ill and the other parts of the unit, the center region of the electrode is cupped toward the upper crystal face effectively to reduce the average length of the air gap llb. Conversely as the temperature decreases from the value at which the electrode l l is undistorted, the center region of the electrode is cupped away from the upper crystal face to effectively increase the average length of the air gap l lb. Here also, the electrode should be properly designed to effect the degree of deformation required to produce the necessary amount of compensation over the desired temperature range.

From the above explanation, it will be understood that different electrodes, deformable in opposite senses and varying amounts relative to the crystal face in response to like temperature changes, amounts may be required to effect complete compensation of crystal units employing crystals and diaphragm units which together are characterized by different temperature-outin association with the upper face of the crystal, 7

the degree of distortion should be such as to produce a temperature-output frequency characteristic D if the particular crystal were clamped between the thermally deformable electrode and a conventional non-temperature responsive electrode. In such case the action of the deformable electrode ll, tending to produce a change in the output frequency of the crystal in one sense,

put frequency characteristics. In this regard it will be understood that in the production of piezoelectric quartz crystals on a volume basis, crystals ground to the same frequency exhibit different temperature-output frequency characteristics. Moreover, manufacturing discrepancies in the production of the diaphragms and their associated driving units may introduce a further variable factor which must be taken into account in accurately compensating the units against frequency drift. Accordingly, in the manufacture of crystal units embodying the present invention on a production basis, it is desirable to provide thermally deformable electrodes ll having different temperature-deformation characteristics falling in certain predetermined classes. By this expedient matching of the electrodes on an individual basis with selected crystals and diaphragm assemblies may be easily and rapidly accomplished to insure the production of crystal units which are substantially completely compensated against frequency drift. Preferably the electrodes are so designed, classified and matched that they are perfectly flat and unstresed at a '7- normal temperature of approximately 72 F. and in conjunction with the matched crystal, provide the exact desired output frequency of the unit at this temperature with no voltage applied to the winding 24. As thus designed, the electrodes will be deformed in opposite directions, 1. e. toward and away from the upper crystal face in response to departures of the temperature in opposite senses from the predetermined value of 72 F.

As previously indicated, energization of the field structure winding 24 by a voltage of pre-' determined magnitude and polarity produces corresponding change in the attractive force exerted by the field structure upon the diaphragm i2. Thus, if the winding 24 is energized by current of the proper polarity to increase the flux traversing the field structure and diaphragm, the attractive force acting upon the diaphragm is increased to move the center region of the diaphragm away from the crystal face and thus increase the output frequency of the crystal unit. Conversely, if the winding 24 is energized by current of 'the proper polarity to decrease the flux traversing the field structure and diaphragm, the force acting upon the diaphragm is partially relieved such that the center region thereof is moved toward the lower crystal face to decrease the output frequency of the unit. In either case, the extent of the frequency change is directly related to the magnitude of the current fiow through the winding 24. For example, in a specific embodiment of the present invention, the characteristic curve A was obtained. This curve shows the crystal unit output frequencies obtained when voltages of different values and opposite polarity are applied to the winding terminals 25. As shown by this curve, the application of a variable voltage of one polarity to the terminals 25 resulted in anincrease in the output frequency of the unit approaching a limit of 5000 cycles. Conversely, the application of a variable voltage of opposite polarity to the terminals 25 resulted in a correspending decrease in the output frequency of the Y unit. I Moreover, tially linear in the the characteristic is substanregion between plus and minus 3000 cycles. This deviation is adequate when multiplied 5 to 6 times for narrow band frequency modulation transmission, such, for example, as in police communication work where deviations of :15 kc. are commonly employed. Thus, the unit lends itself admirably to the production of an oscillator modulator unit adapted for use in a frequency modulation wave signal transmission system.

A system of this character is illustrated in Fig. 6 of the drawings as comprising an oscillatormodulator stage 30, and a power amplifier stage 3| connected in tandem in the order named and arranged to deliver frequency modulated signal carrier energy to an antenna-ground circuit 22. In the system, the present improved crystal unit is employed in conjunction with an electron discharge tube 30a of the pentode type to develop a frequency modulated signal carrier which is transmitted through the compound coup'led fre-' quency selective circuits amplifier tube Me which Preferably, frequency multiplication is obtained in the coupling system conas defined in the 36 and 31 to a power is utilized to amplify the signal carrier and deliver the same through the selective circuit 39 to the antenna-ground circuit 32 for radiation.

necting' the output electrodes of the tube 300 with 1 the input electrodes of the amplifier tube 3la. Such multiplication may be easily obtained by means of a small battery 34 and trdde associated tuning the selective circuits 33 and 31 to a desired harmonic of the output frequency of the crystal I0 and by correspondingly tuning the selective circuit 39 included in the output circuit of the amplifier tube 3|a. Frequency modulation of the developed signal carrier eflected through operation of the described apparatus is obtained by coupling the winding 24 of the crystal unit to the output terminals of a microphone 32. Specifically, this microphone is battery powered by is coupled to the winding 24 through an audio transformer 33.

With this arrangement, audio voltages developed through operation of the microphone 32 are applied across the terminals of the winding 24 through the transformer 33 to effect movement of the diaphragm electrode l2 relative to the lower face of the crystal ill in a manner fully apparent from the preceding explanation. Such movement of the diaphragm electrode l2 results in corresponding changes in the output frequency of the oscillator-modulator stage 30. In other words, the signal carrier generated in this stage is frequency modulated in accordance with the center frequency of the generated frequency-modulated carrier remains exactly under any and all operating encountered.

7 While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various modifications may be made therein which are within the true spirit and scope of the invention appended claims.

at the desired value conditions normally We claim:

1. A variable frequency piezoelectric crystal unit, comprising a pair of electrodes adapted to have a piezoelectric crystal supported therebetween, one of said electrodes being spaced from one face of said crystal by an air gapand being at least in part movable relative to said crystal in response to ambient temperature variations to control variations in the output frequency of said unit with said ambient temperature variations, the other of said electrodes being spaced from the other face of said crystal by a second air gap and being at least in part movable relative to said crystal to vary the output frequency of said unit, and electro-responsive means for moving said other electrode relative to said crystal.

2. A variable frequency piezoelectric crystal unit comprising a piezoelectric crystal, an elecwith one face of said crystal and spaced from said one crystal face by an air gap, electro-responsive means for producing relative movement between said electrode and said crystal to vary the length of said air gap, thereby to vary the output frequency of said unit, said electroresponsive means coasting with said electrode to in part determine the temperature-output frequency characteristic of said unit, and temperature responsive means comprising an electrode associated with the other face of said crystal ambient temperature crystal, thereby to vary the output frequency of,

said unit, said electro-responsive means coasting with said electrode to change the temperatureoutput frequency characteristic of said unit in one sense, and temperature responsive means associated with the other face of said electrode to further change the temperature-output frequency characteristic of said unit in the correct sense to render said characteristic substantially uni-' form.

4, A variable frequency piezoelectric crystal unit comprising apiezoelectric crystal, an elecunit comprising a thermally deformable electrode, a deformable diaphragm. a cup-shaped trode associated with one face of said crystal and spaced from said one crystal face by an air gap, said unit having a non-uniform temperatureoutput frequency characteristic, and electro-responsive means for producing relative movement between said electrode and said crystal, thereby to vary the output frequency of said unit, said electro-responsive means coacting with said electrode in the correct sense to render said characteristic more nearly uniform.

5. A variable frequency piezoelectric crystal unit comprising a thermally deformable electrode, a deformable diaphragm, a holder for said diaphragm, a piezoelectric crystal spaced from said electrode and diaphragm by air gaps and clamped between said electrode and saidholder, said electrode being thermally deformable in the correct sense to reduce variations in the output frequency of said unit with ambient temperature changes, and electro-responsive means supported by said holder for moving said diaphragm relative to said crystal, thereby to vary the output frequency of said unit. v

6. A variable frequency piezoelectric crystal unit comprising a thermally deformable electrode, a deformable diaphragm, a holder for said diaphragm, a piezoelectric crystal spaced from said electrode and diaphragm by air gaps and clamped between said electrode and said holder, electro-responsive means supported by saidholder for moving said diaphragm relative to said crystal, thereby to vary the output frequency of said v unit, said electro-responsive means coacting with said diaphragm to in part determine the temperature-output frequency characteristic of said unit, and said electrode being thermally deformable to change the temperature-output frequency of said unit in the correct sense to render said characteristic more nearly uniform.

7. A variable frequency piezoelectric crystal unit comprising a thermally deformable electrode, a deformable diaphragm, a cup-shaped holder for said diaphragm, a piezoelectric crystal spaced from said diaphragm and electrode by air gaps and clamped between said electrode andholder, said electrode being thermally deformable in the correct sense to reduce variations in the output frequency of said unit with ambient temperature changes, and electromagnetic means supported within said holder for moving said diaphragm relative to said crystal, thereby to vary the output frequency of said unit.

8. A variable frequency piezoelectric crystal holder for said diaphragm, a piezoelectric crystal spaced from said diaphragm and electrode by air gaps and clamped between said electrode and holder, electromagnetic means supported within said holder for moving said diaphragm relative to said crystal, thereby to vary the output frequency of said. unit, said electromagnetic means coacting with said diaphragm to in part determine the temperature-output frequency characteristic of said unit, and said electrode being thermally deformable to change the temperatureoutput frequency characteristic of said unit in the correct sense to render said characteristic more nearly uniform.

9. A variable frequency piezoelectric crystal unit comprising a thermally deformable electrode having projecting clamping parts, a deformable diaphragm, a cup-shaped holder for said diaphragm having projecting clamping parts around the rim thereof, a piezoelectric crystal clamped between said parts such that air gaps are provided between the faces of said crystal and said electrode and diaphragm, said electrode being thermally deformable in the correct sense to minimize variations in the output frequency of said unit with ambient temperature changes, and electro-responsive means supported by said holder for movingsaid diaphragm relative to said crystal, thereby to vary the output frequency of said unit.

10. A variable frequency piezoelectric crystal unit comprising a thermally deformable electrode having projecting clamping parts, a de formable diaphragm, a cup-shaped holder for said diaphragm having projecting clamping parts around the rim thereof, a piezoelectric crystal clamped between said parts such that air gaps are provided between the faces of said crystal and said electrode and diaphragm, said electrode being thermally deformable in the correct sense to minimize variations in the output frequency of said'unlt with ambient temperature changes, and electro-magnetic means supported within said holder for moving said diaphragm relative to said crystal, thereby to vary the output frequency of said unit.

11. A variable frequency piezoelectric crystal unit comprising a thermally deformable electrode having projecting clamping parts, a deformable diaphragm, a cup-shaped holder for said diaphragm having projecting clamping parts around the rim thereof, a piezoelectric crystal clamped between said parts such that air gaps are provided between the faces of said crystal and said electrode and diaphragm, and electro-responsive means supported within said holder for moving said diaphragm relative to said crystal thereby to vary the output frequency of said unit, said electro-responsive means coacting with said diaphragm to in part determine the temperature-output frequency characteristic of said unit, and said electrode being thermally deformable to change the temperature-output frequency of said unit in the correct sense to render said characteristic more nearly uniform. I

12. A variable frequency piezoelectric crystal unit comprising a housing having an apertured crystal receiving cavity therein, a tubular holder extending through said aperture and having a shoulder supported upon the bottom of said cavity, a diaphragm supported by said holder within said cavity, a thermally deformable electrode within said cavity, a piezoelectric crystal spaced diaphragm relative to said holder, thereby to vary the output frequency of said unit.

13. A variable frequency piezoelectric crystal unit comprising a housing having an apertured crystal receiving cavity therein, a tubular holder extending through said aperture and having a shoulder supported upon the bottom of said cavity, a diaphragm supported by said holder within said cavity, a thermally deformable electrode within said cavity, a piezoelectric crystal clamped between said electrode and said holder, said electrade and thediaphragm supporting end of jsaid holder having projecting clamping parts bearing against said crystal which serve to. provide air gap spacings between the faces of said crystal 12 and said electrode and diaphragm, said electrode being thermally deformable in the correct sense to minimize variations in the output frequency of said unit with ambient temperature changes, and electro-responsive means supported within said holder for moving said diaphragm relative to said holder, thereby to vary the output frequency of said unit.

, aooaa .r. PIERCE.

REFERENCES crrsn The following references are of record in the file of this patent:

I UNITED STATES PATENTS Number Name Date I 1,785,036 'MorrisOn Dec. 16, 1930 1,933,735 Hund Mar. 7, 1933 1,962,211 Osnos et a1 June 12, 1934 Ehret et al July 7, 1942 

